1. Field of the Invention
This invention relates to an improved FM-AM converter system in which the FM signal is first changed into an amplitude modulated FM signal, and a separate carrier signal is then modulated by the envelope of the amplitude modulated FM signal.
2. The Prior Art
It has been known in the past to convert an FM signal into an AM signal by applying the FM signal at a constant amplitude to a filter having a montonically sloping characteristic, which is preferably linear with respect to frequency over a frequency range necessary to respond to all frequencies within the band of the FM signal. The effect of such a filter on the FM signal is to change that signal into an amplitude modulated FM signal in which the amplitude modulation envelope corresponds to the signal that produced the original frequency modulation. The envelope of amplitude modulated signal can be demodulated by an amplitude demodulator, or detector, to retrieve the original modulating signal. This original signal can then be used to modulate the amplitude of another carrier to achieve amplitude modulation without the accompanying frequency modulation (except to the extent that amplitude modulation inherently affects the frequency of the carrier).
It is expected that the instantaneous frequency of the frequency modulated signal will have a certain value, which may be referred to as f.sub.o, when the amplitude of the modulating signal is zero. As the modulating signal swings positive and negative about its zero axis, the frequency modulated carrier swings between frequency values above and below the nominal frequency f.sub.o.
However, the frequency f.sub.o may drift or shift from its proper value so that when the FM signal is applied to the filter that has a sloping characteristic, the peaks of each of the individual cycles of the resulting amplitude modulated FM signal will all be shifted to a greater or a lesser amplitude than the desired amplitude, depending upon whether the frequency f.sub.o shifts positively or negatively. This causes the amplitude demodulated signal, which normally is offset from a zero value by a certain amount, to be offset a greater or lesser amount. This is particularly disadvantageous if the offset is toward a lesser amount because negative peaks of the demodulated signal will then approach the zero amplitude value more closely. If the offset is sufficient, when this demodulated signal is used to amplitude modulate the second carrier to produce the final AM signal, the resultant Am signal will be distorted during those instands when the modulating signal is at such negative peak values.